AC Tonnage Sizing Calculator
Quick AC sizing — climate zone × insulation × sun × windows × occupants → recommended tonnage (1, 1.5, 2, 2.5, 3, 3.5, 4, 5 ton).
8 ft standard; >9 ft flagged
Total window area across all walls
Each adds ~600 BTU
Result
- Base BTU (zone)
- 44,000
- Insulation factor
- ×1
- Sun factor
- ×1
- Window adjustment
- +600 BTU
- Occupant adjustment
- +2,400 BTU
- Kitchen adjustment
- +1,200 BTU
- Total cooling load
- 48,200 BTU/hr
- Raw tonnage
- 4.02 tons
- Recommended sizeThis is a quick estimator. Manual J load calc is required for permit + code submittal.
- 4 ton AC
This estimate is based on national average costs and may vary by region, project specifics, and market conditions. Use as a starting point for your bids.
Quick AC sizing — the rule of thumb
Multiply conditioned sq ft by a climate-zone BTU/sf factor, then adjust for insulation, sun, windows, and occupants. The calculator above does the math; this section explains the factors so you can sanity-check.
Climate zone BTU baselines
| Zone | BTU / sq ft | Example cities |
|---|---|---|
| Zone 1 (hot-humid) | 26 | Miami, Houston |
| Zone 2 (hot) | 24 | Phoenix, Tampa |
| Zone 3 (warm) | 22 | Atlanta, Dallas, San Diego |
| Zone 4 (mixed) | 20 | DC, Kansas City, Boulder |
| Zone 5 (cool) | 18 | Chicago, Boston, Salt Lake |
Why this isn't Manual J
This is a quick estimator for the early ballpark conversation with a homeowner. Permit-required, code-submittal, and Energy-Star certifications all require Manual J — which accounts for window orientation, infiltration ACH50, duct losses, internal gains (lighting + electronics), and design temperature differentials. Manual J runs $200-500 from any HVAC design service. Use this calculator to sanity-check before paying for the formal load calc.
Oversize warning
The #1 residential AC mistake is oversizing. An oversized unit hits temperature setpoint fast, then short-cycles — never running long enough to dehumidify. The customer ends up with cold AND clammy air, files a complaint within the first humid season, and the contractor is on the hook for a swap-out. Right-sized units run longer cycles, dehumidify properly, and the customer is happy.
Frequently asked questions
How many tons of AC do I need per square foot?
Rule of thumb: 1 ton per 500-600 sq ft in hot climates, 1 ton per 600-700 sq ft in cooler climates. A 2,000 sq ft home in Atlanta (zone 3) typically needs 3-3.5 tons. Manual J is the formal load calc and always wins for permits — this calculator approximates the rule of thumb adjusted for climate, insulation, sun exposure, windows, and occupants.
What's the difference between BTU and tons?
12,000 BTU/hr = 1 ton of cooling. So a '3-ton' AC is rated at 36,000 BTU/hr at standard conditions. The rating drops at higher outdoor temperatures (a 3-ton at 95°F outdoor pulls less than at 75°F). Always verify the unit's AHRI-rated capacity at your design temperature for accurate sizing.
Is bigger always better?
No — oversizing is the #1 residential AC mistake. An oversized unit cools the temp set-point fast then shuts off, but doesn't run long enough to dehumidify. Result: cold AND clammy. Properly-sized units run longer cycles, hit the temp setpoint AND dehumidify. Aim for the right size, not the biggest.
Do high ceilings change the math?
Yes — a 9 ft ceiling adds 12% more volume to condition than 8 ft. Vaulted ceilings (12+ ft) add 50% or more. The calculator above adjusts the base BTU/sq ft but flags ceilings above 9 ft as needing a manual-J load calc.
When do I need a Manual J load calc instead?
Always for permitted work, code submittal, and Energy Star certification. Manual J accounts for window orientation, infiltration, duct losses, internal gains, and design temperature differentials — none of which this quick estimator does. Manual J costs $200-500 from an HVAC contractor or design service. Treat this calculator as a sanity-check, not a substitute.
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